Light responsive apparatus



y 22, 1958 D. R. CRAIG 2,844,738

LIGHT RESPONSIVB APPARATUS Filed June 19. 1956 .5; LLI

m r wr f 5 INVENTOR I Dwm R. CRAIG Unite LIGHT RESPONSWE APPARATUS Application June 19, 1256, Serial No. 592,437

6 Claims. (Cl. 250-214) This invention relates to light responsive apparatus eminently suited for use in the fields of photography, nucleonics and radiography.

There are many instances in which it is desirable to integrate the amount of light arriving at a particular point, and at a predetermined value perform an operation in response thereto. It is also highly desirable in a number of applications to continuously measure variations in light intensity at such a point during the integrating operation. Specific examples of such applications of the present invention will be illustrated in an application entitled Photographic Method and Apparatus filed by the present inventor concurrently herewith.

It is among the objects of the present invention to provide a light responsive switch comprising a photosensitive device having an anode and a cathode, a capacitor having a first plate connected to the anode and a second plate maintained at a ground potential, a charging circuit connected to the first plate including a normally open switch and a potential source, a trigger circuit connected to the first plate including a variable voltage source, a discharge device and a resistor in series, and an output circuit connected to the trigger circuit intermediate the discharge device and resistor. The discharge device preferably assumes the form of a gas discharge tube and the photosensitive device preferably possesses pentode characteristics and may assume the form of a photomultiplier tube.

The output circuit may contain a relay which will in turn energize the load to be controlled or actuated. A second resistor will be interposed between the anode and discharge device and a second output circuit containing a capacitor will be connected between the second resistor and the anode where it is desired to obtain a response to instantaneous variations in the intensity of the light being integrated.

A more complete understanding of the invention will follow from a description of the accompanying drawing which assumes the form of a circuit diagram.

A photomultiplier tube 10 having an anode 12, a cathode 14 and dynodes 16 is interposed in the path of light rays 18 directed from a suitable source. By way of example, the source may be a cathode tube as disclosed in 7 the companion application mentioned above, or otherwise.

The anode 12 is connected through a resistor 20 to parallel circuits having a common terminal 22. The upper leg of the parallel circuit as depicted in the drawing contains a normally open switch 26 leading to a fixed positive voltage source 27, while the lower leg contains a gas discharge tube such as a neon tube 28 in series with a resistor 30 connected to a variable source of positive voltage 32 designated in the drawing as an exposure factor control. In efiect, this voltage source 32 permits variation in the preset value of integrated light by producing any desired voltage value within a desired range. Interposed between the common terminal 22 of the parallel circuit and a ground 34 there is an integrating capacitor 36. And tapped between the discharge device 28 and resistor 30 of the lower branch of the parallel circuit, there is an output 3135 Piltfi r 2,844,738 Patented JulyZZ, 1958 price Interposed between the anode 12 and the.resistor.20,. another output circuit is provided comprising a lead 44 and a capacitor 46, the output lead 48 of this circuit supplying an indication of variations in the intensity of the light rays 18 at any particular time.

In the event that there is no need for an instantaneousresponse to variations in light intensity, the' resistor 20, leads 44 and 48 and capacitor 46 will bunnecessary in which event the anode12 can be connected direct to the common terminal 22. 1

To initiate an integrating cycle, the Switch '26 will be closed to charge the capacitor 36 to the fixed voltage V, of the source 27; Then the switch 26'will be opened and integration of light will begin. 'During this period of integration, the only discharge path for the capacitor 36; will be through the resistor 20 and photomultipliertube 10. When the light intensity I is zero, there is no discharge current except for phototube dark current and other leakage currents which are relatively insignificant.

Also, there is no charging current under such circumstances since the switch 26 is open and the gas discharge tube 28 in its unfired condition is likewise essentially open.

When light falls on the cathode 14 however, photomultiplier current will discharge the capacitor 36 at a rate proportional to light intensity, independent of the anode voltage, for most practical purposes. The voltage variation developed across the resistor 20 by the phototube current i will be tapped ofi through the capacitor 46 to provide a continuous measure of instantaneous variations in light intensity.

Since the capacitor 36 is the source of the photomultiplier tube. current, average light intensity governs the average rate of discharge according to the expression:

E I-1{.12-.Z{ZAz Inasmuch as a removal of charge is accompanied by a corresponding change in capacitor voltage according to the expression:

Q=C AV where C is the capacity of the capacitor 36, it will follow that after a time T, the capacitor voltage will have dropped sufficiently below +V of source 32 to fire the gas discharge tube. The relation then becomes:

At IT wherein the change in voltage across the capacitor 36 is directly proportional to the integrated amount of light reaching the photomultiplier tube.

Accordingly, the number of incident photons will be directly proportional to the number of units of charge removed from the capacitor 36.

When the discharge device 28 fires, a voltage pulse will be developed across the resistor 30 for transmission by the leads 38 to actuate the switch or relay 40 to in turn perform the operation desired which may be shutting off the light source, actuating a counter, or other type of control.

By virtue of the circuit of the present invention, stray leakage paths which would lead to erroneous integration are minimized by using an isolated capacitor to supply anode voltage to the photomultiplier tube and by using a gas discharge tube characterized by an extremely high resistance when unfired to detect the triggering voltage of the integrating capacitor and then to provide a sharp trigger pulse, schematically indicated by reference character 50, upon firing. While a negative pulse will be derived from the circuitry shown, a positive pulse can be derived, if preferred, by interchanging the positions of discharge device 28 and resistor 30. A wave form 5 2 is symbolic of theoutput tapped off between the anode 12'and re-' sister. 20..where it is desired to obtain an-indication of light intensity variations. Y

The form of the invention thus described and illustrated should .not :be construed as limiting beyond the scope of the appended claims. i I I i l claimz'. v l 1. A light responsive switch comprising a photosensitive device having an anode and a cathode, a capacitor having a first plate connected to said anode and a second platemaintained at a' ground potential, a charging circuit connectedto said first platejincluding a normally open switch and a potential source, a trigger circuit connected to said first plate including a variable voltage source, a discharge device-and a resistor in series, and an output circuit connected to' said trigger circuit intermediate said discharge device and resistor, said discharge device having a non-conducting condition of substantially constant high resistance isolating said capacitor against discharge therethrough."

a 4 2. A light responsive switch as set forth in claim 1 wherein said discharge device is a gas discharge tube.

3. A light responsive switch as set forth in claim 1 wherein said photosensitive device possesses pentode characteristics.

4. A light responsive switch as set forth in claim 1 wherein said photosensitive device is a photomultiplier tube.

5. A light responsive switch as set forth in claim I wherein said output circuit contains a relay.

6. A light responsive switch as set forth in'claim I wherein a second resistor is interposed between said anode and discharge device and a second output circuit containing a capacitor is connected between said second resistor and said anode to respond to instantaneous variations in light intensity.

References Cited in the file of this patent UNITED STATES PATENTS Twyman Dec. 12, 1933 Fuller May 7, 1935 

